Cytokinesis is a critical event in which the cytoplasm and plasma membrane are partitioned into two cells. Key unresolved questions regarding its molecular mechanism include I) how the cleavage plane is positioned; ii) how furrow components are recruited to the plasma membrane; and iii) how changes in the cytoskeleton are coordinated with the cell cycle. Understanding these questions in terms of cytokinesis may be important in revealing the molecular mechanisms of other contractile processes such as morphogenesis. This proposal focuses on two approaches. First, the application of a series of reagents directed against furrow components will be used to evaluate their behavior during aberrant cytokinesis in Drosophila and yeast mutants. Second, analysis of the molecular mechanisms of septin function, a phylogenetically widely conserved family of proteins that function during cytokinesis will be performed in vitro and in yeast and flies, with focus on identifying the proteins that bind to the septins in a nucleotide dependent fashion.